There is no hard and fast rule.
In some applications, your users might want to open one form, then minimize
it, and then while on the switch board, launch another form....perhaps even
do this a 3rd time.
..
However, if you have a serious of forms that need to be completed, then
obviously you don't want you users retuning to the switch board without
FIRST close the form they are on....
In most, (if not all) of my applications, I don't hide the previous form,
but a VERY large portion of my forms are model. That means that I might
have a button on the form that opens another form. However, the user can NOT
return back the way the came unless they close the existing form (they have
to do this since my forms are set as modal). Hence, they continue this
process of closing the form, and returning all the way back to the
switchboard.
You can also consider a design where each form is model, and also maximised.
This means that the current open form will ALWAYS cover up the previous
form.
In other cases, I tend to just size the forms so that they cover *most* of
the previous form.
So, I think the "main" issue would be do you want to allow several forms
open, and can the user switch between them, *or* must they close the current
form they are on to return to the previous form...